Chemo-poroplastic analysis of a borehole drilled in a naturally fractured chemically active formation H. Roshan n , M. Fahad School of Petroleum Engineering, University of New South Wales, Australia article info Article history: Received 25 February 2011 Received in revised form 8 February 2012 Accepted 6 March 2012 Available online 29 March 2012 Keywords: Chemically active rock Naturally fractured shale Osmotic flow Plasticity abstract A coupled chemo-poroplastic model is developed to investigate the change in ion transfer and its effect on pore pressure and effective stresses in chemically active fractured media. The equivalent perme- ability tensors are calculated using the boundary element method for an arbitrary oriented fractured system, and used in the chemo-poroplastic model. An explicit modified Euler algorithm with substepping including a yield surface correction scheme is used to integrate the plastic stress–strain relation. Super-convergent patch recovery method is also used to accurately evaluate the time dependent nodal stress tensors from the stress tensors of gauss points. From the results of this study it was revealed that the pore pressure drops around the wellbore due to chemical osmotic back flow. The drop in pore pressure, however, becomes a function of fractures’ orientation and density. The state of stresses reach the yield strength of the rock in fractured shale, thus forming a plastic zone around the wellbore. The relaxation of effective tangential stresses in the plastic zone, therefore, has a potential to cause stability problems. & 2012 Elsevier Ltd. All rights reserved. 1. Introduction Many shale formations are naturally fractured and characteristic properties of these fractures can vary widely [1–3]. In previous wellbore stability analysis, however, shale formations have been treated as a homogeneous rock which falls short in describing the fractured shale behaviour. The first experimentally verified fully constitutive model for hydration swelling of water active rocks under isothermal condition was proposed by Heidug and Wong [4]. Ekbote and Abousleiman [5] used a different approach, which is based on the extended version of equations of poroelasticity and the work of Sherwood et al. [6], to solve the problem of chemo- poroelasticity in transversely isotropic porous media. In a recent work the effect of ion advection on pore pressure and stress changes around a wellbore drilled in relatively high permeable shale forma- tions were investigated by Roshan and Rahman [7]. The authors also investigated the plastic behaviour of homogeneous shales under thermal, chemical, hydraulic and mechanical interactions [8]. Though, an attempt has been made to model the chemo- poroelasticity in fractured shales, the proposed model does not take into account the fractures explicitly [3] and the plastic behaviour of shale rocks is not considered. The presence of arbitrary oriented fractures, with variable length and spacing, makes it difficult to assess changes in pore pressure due to chemical and hydraulic flow interactions. Thus several methods such as single and dual continuum and discrete approaches have been proposed to overcome the difficulties of simulation of naturally fractured reservoirs (NFRs) [9–11]. Although the single and dual continuum approaches are effective in fluid flow simulation, the properties of fractures such as geometry and orientation are not taken into account and individual fractures are not treated explicitly [11]. Among the different forms of discrete fracture approaches a hybrid scheme has been widely used in which the effective permeability tensor concept has been introduced to simulate the fluid flow [12,13]. The effective permeability tensor approach also provides a better computational efficiency than the other discrete fracture models [11]. In this study the authors extended their previous works [7,8] to investigate the effect of fractures on solute and fluid transport and its effect on effective stresses around the wellbore. To achieve this, the concept of permeability tensor of an arbitrarily oriented naturally fractured shale as well as governing equations of chemo-poroplasticity are introduced and the results of solute transport, pore pressure and stress distributions around the wellbore are presented and discussed. 2. Permeability tensor estimation Teimoori et al. [11] proposed an efficient numerical method to estimate the grid based permeability tensors for an arbitrarily Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/ijrmms International Journal of Rock Mechanics & Mining Sciences 1365-1609/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmms.2012.03.004 n Corresponding author. E-mail addresses: hamidrooshan@yahoo.com, Roshan@nigc.ir (H. Roshan). International Journal of Rock Mechanics & Mining Sciences 52 (2012) 82–91